Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

Gneiss is a high-grade, foliated metamorphic rock. It differs from lower grade foliated metamorphics, such as slate, phyllite, and schist, by having alternating bands or stripes of light and dark minerals. Metamorphism has been so intense that minerals have self-segregated into discrete zones. Most gneisses form by metamorphism of schists or granites.